Stable isotopes mass dependent fractionation

Equations (8) and (10) are applicable to stable isotope systems where isotopic fractionation occurs through mass-dependent processes which comprise the majority of cases described in this volume. These relations may also be used to identify mass-independent fractionation processes, as discussed in Chapter 2 (Birck 2004). Mass-dependent fractionation laws other than those given above distinguish equilibrium from kinetic fractionation effects, and these are discussed in detail in Chapters 3 and 6 (Schauble 2004 Yormg and Galy 2004). Note that distinction between different mass-dependent fractionation laws will generally require very... 

Johnson TM (in press) A review of mass-dependent fractionation of selenium isotopes and implications for other heavy stable isotopes. Chem Geol... 

The origin of mass-dependent fractionation (MDF) in isotope systems lies in the mass dependence of the molecular properties (e.g., zero-point energy) and physical processes (e.g., evaporation) affecting the compound. If a compound comprised of atoms with 3 or more stable isotopes, such as oxygen or sulfur, deviates from a mass-dependent relationship, the compound is said to exhibit mass-independent fractionation (MIF). MIF signatures are not affected by mass-dependent processes, and so are excellent tracers of the small number of mass-independent processes that exist in nature. 

Most common is the process of mass-dependent fractionation, in which the stable isotope ratio is altered as the consequence of physical processes differentially affecting atoms or molecules of different mass. Isotopes are fractionated relative to one another according to thermodynamic, kinetic, and diffusion processes. A simple example is the way in which oxygen isotopes in water molecules are fractionated during the process of evaporation. Water molecules containing the lower mass isotope leO are more likely to become water vapor than those containing the higher mass isotope lsO. Hence the water vapor is enriched in isotope leO and the liquid water is enriched in isotope lsO. 

In a subsequent investigation, the same group [14] utilized the technique developed to study spedes-specific stable isotope fractionation of mercury during methylation of IHg by anaerobic bacteria Desulfobulbus propionicus) in the dark (Table 17.1). The isotopic composition of Hg in IHg and MeHg species was measured on-line from the same sample. The authors demonstrated mass-dependent fractionation of Hg isotopes during the experiment. 

Units which are used in isotopic work depend on the precision of the measurements. Generally 5 units are used for stable isotopes and correspond to permil relative deviation. It is used occasionally also for non linear effects and then they are permil (%o) deviations without reference to mass differences between the isotopes. Since the beginning of the 70s (e.g., Papanastassiou and Wasserburg 1969) thermal ionization data are often given in e units which are fractional deviation from the normal in 0.01%. With the new generation of more precise instruments, results are sometimes given in ppm (parts per million) relative to a terrestrial standard sample. 

The major analytical complication in Mo isotope analysis is precise correction for isotope fractionation during Mo purification and mass spectrometric analysis. This subject is reviewed in general by Albarede and Beard (2004), and is discussed here in particular reference to Mo. It is important to recognize that this challenge is fundamentally dififerent in mass dependent stable isotope studies as compared to investigations of mass-independent Mo isotope variations produced by nucleosynthesis. The latter have received attention in recent years for high-precision determination of Mo isotope composition (e.g., Dauphas et al. 2002a,b Yin et al. 2002), but are not relevant here. 

Heidenreich JE, Thiemens MH (1983) A non-mass-dependent isotope effect in the production of ozone from molecular oxygen. J Chem Phys 78 892-895 Helman Y, Barkan E, Eisenstadt D, Luz B, Kaplan A (2005) Fractionation of the three stable oxygen isotopes by oxygen producing and consuming reactions in photosynthetic organisms. Plant Phys (2005) 2292-2298... 

This phenomenon forms the basis for the formulations of Urey (1947) and Bigeleisen and Mayer (1947) for the temperature dependence of isotopic exchange between two molecules. With the nearly simultaneous development of the isotope-ratio mass spectrometer by Nier et al. (1947), the potential for application of stable isotopes was created. Other isotopic fractionation processes are observed in kinetics, diffusion, evaporation-condensation, crystallization, and biology (e.g., photosynthesis, respiration, nitrogen fixation, sulfate reduction, and transpiration). The concomitant isotopic fractionations can also be used to provide details of the relevant process. 

Oxygen consists of three stable isotopes 0, " O and " O. For most applications only the ratio between the more abundant Lso-topes, 0 and O, is measured. If all fractionation processes in the environmental system were purely mass-dependent, measurements of 0/ 0, would be redundant, as they could be predicted from the 0/ 0 ratio. However, it has recently been ob.served that photochemical exchange between O2, Oi, and CO, in the stratosphere involves mass-independent fractionation among the oxygen isotopes (1990 Thiemens et al., 1993a, b Thiemens, 1999). Thereby O2 becomes anomalously depleted, while CO2 becomes anomalously enriched. Because of this, measurements of 0/ 0 in atmospheric O2 and/or CO2 provide an independent piece of information. Conveniently, one may define an O anomaly tracer (A 0) (Thiemens et n/., 1995b)... 

The stable isotopic compositions of elements having low atomic numbers (e. g. H, C, N, O, S) vary considerably in nature as a consequence of the fact that certain thermodynamic properties of molecules depend on the masses of the atoms of which they are composed. The partitioning of isotopes between two substances or two phases of the same substance with different isotope ratios is called isotopic fractionation. In general, isotopic fractionation occurs during several kinds of physical processes and chemical reactions ... 

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